Cellular communication systems have traditionally been based on circuit switched connections which facilitate real time communications that are sensitive to delays and jitter. However, increasingly cellular communication systems utilise packet switched communication to support both real time and non-real time communications.
For example, for the 3rd Generation Partnership Project (3GPP) Universal Mobile Telecommunication System (UMTS) includes an interface between the Radio Network Controllers (RNC) and a circuit switched network which is known as the Iu-CS interface. Despite the interface relating to circuit switched communications, it has been proposed that the Iu-CS interface can be implemented using packet switched technology and in particular Internet Protocol (IP) based transport protocols, such as the Real Time Protocol (RTP). Thus, it has been proposed to implement the Iu-CS interface using an IP packet switched network using real time protocols such as RTP to support the underlying real time communications.
In such systems, a Media GateWay (MGW) is typically implemented to interface between the packet switched network and the circuit switched network. The MGW comprises a jitter buffer which buffers the packetized real time data received from the RNCs such that the data can be provided as a real time stream to the circuit switched network. The RTP protocol specifically uses a timestamp applied by the sender to retime the circuit switched data at the MGW. The RTP functionality of the MGW also uses the timestamp to measure network jitter and also discards data packets that exceed a jitter corresponding to the jitter buffer depth.
However, this approach has a number of associated disadvantages. In particular, when a handover of a communication occurs resulting in a handover from one serving RNC to another (known as a UTRAN SRNS relocation), the RNC responsible for generating the uplink transport bearer stream transmitted to the MGW is moved from the original source RNC to the new target RNC. As the RNCs are not synchronized, the target RNC will use a timestamp which is not synchronized with the timestamp of the source RNC and accordingly the MGW will receive data packets with a discontinuity in the timestamps. To the MGW this will appear as a potentially large variation in the delay (which could even be negative) resulting in an uplink data loss. Thus, an apparent gap in the uplink data provided to the circuit switched network may occur when the timestamp source used by the RTP protocol changes from that of the source RNC to that of the target RNC resulting in either re-synchronization of the MGW jitter buffer (to the target RNC) or apparent jitter at the MGW which exceeds the jitter buffer depth (for a 3GPP system the MGW does not forward uplink data until the new timestamp is aligned with the old).
In order to address these disadvantages it is possible to closely synchronise the time bases used for timestamping between all RNCs. However, achieving such synchronisation is very complicated, impractical and results in complexity and cost increases which are undesirable as well as in reduced reliability of the system.
Hence, an improved system would be advantageous and in particular a system allowing facilitated implementation, reduced complexity, improved handover performance, reduced data loss, increased reliability and/or improved performance would be advantageous.